Zone-melting crystallization technique



i W. R. WILCOX ZONE-MELTING GRYSTALLIZA'ION TECHNIQUE Filed Aug. 2. 1961 M f im ATTORNEY.

United States Patent O 3,189,419 ZONE-MELTNG CRYSTALLIZATION TECHNIQUE William B. Wilcox, Torrance, Calif., assignor to The United States of America as represented by the United States Atomic Energy Commission Filed Aug. 2, V196i, Ser. No. 128,907 4 Claims. (Cl. 23-301) The present invention relates to a method and apparatus for the accomplishment of more effective mixing in the solution phase of fractional crystallization processes and more particularly to the generation of free convection currents to achieve solution mixing in those zone-melting processes fwhere -mechanical stirring is not feasible. The benefits of .this invention will .be Widelyv realized as zonemelting has found valuable application in research and manufacture .for purifying metals, semiconductors, organic and inorganic chemicals, particularly in the field of solidstate electronics.

By zone-melting is meant that group of processes which achieve the controlled distribution of solutes or soluble impurities in crystalline materials by ythe slow movement of one or more short molten zones along a relatively lengthy charge of a crude crystalline solid. Each molten zone has two .solid-liquid interfaces: a melting interface alt the leading edge ofthe zone and at which the crude solid is melted, and a freezing interface at the trailing edge of the zone at which solid of an altered solute concentration is frozen out, Within the zone, the crude melted solid (a binary solute-solvent system) mixes with the contents of the zone and thus reaches the freezing interface. At the freezing interface solute which lowers the melting point of :the solvent .is rejected by the rfreezing solid and remains in the liquid; conversely, solute which raises the melting point -of the solvent is accepted at the freezing interface and so accumulates in the solid, consequently lessening its concentration in the liquid. Thus, it is at the freezing interface that the rejection or acceptance of the solute occurs, and as in more conventional fractional crystallization, .the eiciency of this distribution is largely dependent Ion the amount of stirring which -is effected in the liquid portion yof .the system, particularly that liquid in the immediate region of the growing crystal surface.

The distribution of solute in fractional crystallization processes is described by a parameter of the material, the equilibrium distribution coefficient, defined as the ratio of solute concentration in the solid, Cs, to that in the adjacent liquid C1, `and designated by the symbol ko. The value of ko is greater or lesser than one, depending on whether the solute raises or lowers, respectively, the melting point of the solvent, and its magnitude may be read from a phase diagram. In actual practice however, .and this includes zone-melting, freezing does not occur slowly and liquid mixing is not complete. Under conditions where ko is greater than one, solute is rejected by the growing crystal faster than it can diffuse into the liquid and a solute-enriched layer rthus builds up in the liquid ahead of the .freezing interface. Where ko is less than one a solutedepleted layer builds up in an analogous manner. The concentration of solute freezing out of or being accepted by) the solid then is .affected by the solute concentration in this layer rather than in the bulk of the liquid. The equilibrium distribution coefficient defined above is no longer accurate and instead, an effective distribution k must be determined. This latter coecient k is similarly equal to ythe ratio Cs/ C1 where Cs is the solute concentration lin the solid but where C1 is the solute concentration in the bulk of the liquid.

The relation between these coefiicients is described by a model proposed by Burton, P-rim and Slchter [1. A.

3,189,419 Patented .June 15, 1965 fice through this layer, and .solute movement is thus impeded in proportion to the .thickness of this layer, hereinafter called the diffusion layer. Upon reaching the bulk fluid solute is completely .and immediately mixed into the bulk of the zone. At the freezing interface the relationship between solid and liquid concentrations is given by kn, and in `the bulk fiuid as proportional to the freezing solid by k. According to the above model these are related by the following equation:

ko-l- (l-ca) exp.

Dpi) Where:

k is the effective distribution coefficient,

ko is the equilibrium distribution coeficient,

V vis the rate of growth of the solid (or zone .travel rate),

D is the molecular diffusion coefficient of the solute in the liquid,

is the diffusion layer thickness,

ps is the density of the solid, .and

p1 is the density of Ithe liquid.

It is demonstrable, both in practice and in theory, that convection in the liquid Will reduce the thickness of the diffusion layer and thus improve the effective distribution coefiicient. For where zone-melting is intended for purification, as in zone-refining, a large |1-k[ is desired; thus, the value of k should be increased if k is greater than 1 and decreased if k is less than 1. Either of these variations may be achieved, as shown in 4the above equation, by a decrease either in growth rate V or in diffusion layer thickness A decrease in diffusion layer thickness bowever, as accomplished by convection, is vpreferable since fthe process may be carried out with greater efficiency without sacrifice of speed. Conversely, where controlled distribution of solutes is the object of zone-melting, as in zone-leveling, and a constant 'ratio of solute concentration in solid and liquid (k) is desired, a reduction in diffusion layer thickness will allow, within permissible limits, a several 'fold increase in growth rate, thus assuring more rapid production of the desired crystalline material.

Hence, :by whatever convenient means possible, the diffusion layer thickness should be reduced. Where forced convection by mechanical stirring, induced current heating, magnetic forces or liquid pumping is not feasible due to limitations imposed by the apparatus or the materials, .free convection must be relied upon to effect stirring of the bulk fluid, Heretofore effective free convection, other than that which is normally present yto only an insignificant degree, was obtainable only through selection of a particularly shaped molten zone, generally one having a tall, vertical liquid-solid interface.

The present invention provides for the production of free convection which is not only much more effective than that formerly attainable but which is also independent of zone shape. For the reasons described previously,

the result of producing such convection is to increase the separation efficiency and/ or growth rate in zone-melting processes thereby effecting increased eiciencyand a reduction in processing costs. Y

It is an object of the present invention therefore to increase the separation eiciency of zone-melting processes.

4' vection to enectstirring of the liquid.

, Y s g It isanother object of this invention to improve zone.u melting by providing for an increase in production rate.

YIt is another object of this invention to reduce the pro- It is another object of this invention to generate free :convection currents within the molten zone of a zone- Ymelting process in order to produce stirring.

lIt is another object of this invention to reduce the diffusion layer thickness in fractional crystallization procedures of the. zone-melting-class by producing free con i Freezconvection isrthat uid motion which isV caused by density variations resulting from temperature and/or con- VYcentration variations within the fluid as opposed to forced convectionV which is fluid motion caused by externally induced agitation of the liquid. According to the present ilinvention, a thermal differential is produced within the uid of the molten zone, particularly near the freezingv l interface, by the insertion Vof a foreign body having thermal characteristics differing from those of the liquid.

This foreign body, which may be hotter or cooler than the fluid or have a thermal conductivity greater than the iluid,ris placed within the zone-melting apparatus -so that y it passes near the freezing interface.

The temperature, and consequently the density of the'iluid immediately adjacent, this body is made to differ significantly from that Vgrof the'rest of the iuid and currentsrdrue to free convection are thus established.

This foreign body may be of several different types.

"Examples are an electrical wire or wires located near the j freezing interface and heated by the passage of an electrical current, and a tube or coil, similarly positioned, I through which is circulated a hot or cold fluid. A particu- 'flarly advantageous embodiment of this invention, Vin terms ,of convenience and magnitude of results obtainable, is thatV they foreign body be a simple rod or tube and be of ya material which has a thermalconductivity greater than :that of the'uid,'a condition usually met by copper or stainless steel, and which passes into a heat source or sink. Since the Vrecrystallizing solid in the zone-melting process is cooled, this solid itself may serve as a heat sink merelyV by burying'part of the foreign body in the solid. Y Because this material Ydoes conduct heat well, it is cooler fthan the liquid in which it resides; heat is therefore trans- 4.ferred from the liquid to the rod thus causing the desired `temperature differential Within the liquid. The container l which holds the zone-melting material may itself, in some j' instances, be adapted to provideV the rapid heat conduction; This latter arrangement may beV used in conjunction with additional conductors such as axial rods or wires 1 Vinside the container and running through melt and solid.

. VVWithan'y of these structures, the foreign body should l r be of a materialwhich does not dissolve in, react with or Y melt in the components-being processed. The tempera- Vturedifference between this body and the uid must be sufficient to cause the density difference neededrto pro- Vduce appreciable-free convection currents. stances a fraction of a degree of temperature difference In someV inwithin the uid will be effective. It is preferable, however, to have the temperature difference as large as possible within the following limits: if this body is hotter thanfthe surrounding fluid, itmust not be so Vhot as tok Uvaporize the liquid, cause the zone to expand appreciably '.L in'size or cause thermal degradation Aof the liquid; if this body is cooler than the liquid, it must not be cool enough to kfreeze an appreciable amount of liquid around it. In 1the case ywhere the foreign body is cooled by thermalV f conduction a conductivity at least several times that of the Vhuid is lgenerally preferable. Y

'The foreign body must be positioned within the zonei'melting tubev so that at least part of it is always near enough to the freezing interface thattthe convection currents which are generated will impinge on the freezing interface, or draw uid from the neighborhood of this Y tween the foreign body and the freezing interface for the duration of the zone-melting treatment. Y A f This foreign body is distinct from that apparatus, such as immersion heaters, heat transfer tubes and the like, which is immersed in the charge in order to create the molten'zone. As it is the function of such apparatus to provide the'heat necessary to meltV the surrounding material, such apparatusrwill necessarily be positioned in the center of the molten zone rather than very near the freez- Y ing interface and the convection currents so generated will not be effective for lessening the-,diffusion layer thickness as is accomplished by the present invention. Y

The effectiveness of the kforeign body is found by, de-

termining the diffusion layer thickness and the solute separation achieved in a zone-melting runV performed without the presence of a foreign Vbody and againnding `these values under duplicate design Vconditions but where a for- .eign body is included. A comparison of these values will Y show the reduction in diffusion layer thickness and the proportional-increase in either solute separation or travel rate which has been attained. Y

- The interdependencepof the lsolute separation or travel rate and diffusion layer thickness is Vshown in theseveral equations which have been derived for expressing zonemelting results. For example, inY zone-refining the total separation of solute-(impurity) caused by a single zone passage down a very long charge is given by the equation:

where StV is total separationV (total mass in g./cm.2 of Vsolute removed from or added to a crystal of unit cross sectional area),

Cs .is the solid solute concentration at z,

Co is the originally uniform Vsolid solute concentration throughout the charge,

z is the distance traveled by the zone,

L is the length of the zone, and

ko, V, D, ps and 'p1 are as previously defined. Y From the foregoingexpression, it will be apparent that a reduction in diffusion layerV thickness will effect a comparable increase in separation St especially at a high zone travel rate V and conversely, in zone-leveling,` thel zone travel rate may `be increasedV by 'the same `factor by which the diffusion layer thickness is reduced to maintain melting equipment employing the presentin-vention, and Y FIGURE 2 is an enlarged view of the portion of QFIG- URE l enclosed by dashed line 2 thereon and ,showing the convection currents produced by the presentinven- Y tion,'and yto the example which `describes the'essentials of a `Vzone-melting operation utilizing Vthe present invention;

The-apparatus shownV in FIGURE 1 illus-trates the application .of the invention to one lparticular form of zonemelting equipment. Applicants invention however is not limited to this specific arrangement but may be adapted, by those skilled in the art, to the various other equipment setups presently in use Where stirring by free convection may be desirable.

4Referring now to FIGURE 1 there is shown a closed ie f.

hollow upright cylindrical vessel 11 comprising the main body, or zone chamber, of this apparatus. Coaxially disposed within cylinder =11 is .a much smaller sealed tubular container 12 lled with the crude material 15 to be processed. Closely encircling container 12 but not ailixed thereto is a stationary annular heater 13, but in this instance a resistance heater comprising a tube 14 yof a heat conducting material 'about which is wound a heating tape 16. Lead-s 17 from an electric-al power 4supply provide the heating current to tape 16. A cylindrical housing 18 is disposed cylinder 11 sur-rounding heater 13 to form the heating chamber. Disposed coaxially around the cylinder 11, immediately above and below heater housing 18, are hollow annular cooling chamber housings 19 and 19 through which are circulated coolant-s at a temperature suflicient to effect freezing of the zone-melted material 15. A pair of disc-shaped insulating shields 21 and 21' are disposed coaxially within cylinder 1-1 above and below the heater housing 18, the shields each having a central opening through which tube 12 passes. In .addition .to providing heat insulation around housing 1S these shields also serve 4as guides to keep tube 12 centered in its movement through the heating and cooling chambers. Movement of tube i12 is effected by means of a shaft 22 one end of which is attached to a cap 23 fitted over the top of tube 12. Rod 22 is transpierced through the top of cylinder 11 through a bushing 24. A chain 26 is secured to the -outer end of rod 22 to draw the tube 12 upwardly, the chain being engaged on a pulley 25 driven -by a constant speed electrical motor 30 through a speed reducing gear box 27 The desired thermal differential within the molten zone of material is provided by foreign body 28 positioned axially within tube 12. In this embodiment, the foreign body comprises a long hollow tube 28 of a highly conductive, non-reactive material -such as stainless steel and has an outer diameter small in comparison to the inner diameter of tube 12 and a length substantially the same as that of tube 12. The length of foreign body 28, which length corresponds to the total zone travel length, thus assures effective contact between this body and the freezing interface for the duration of the zone-melting run.

Referring now to FIGURE 2, which is an enlargement of the area shown within the dashed line 2 of FIGURE 1, there is shown the eect of the foreign body 2S in the molten zone. Shown in FIGURE 2 are tubular container .12 filled with the crude material 15 to be processed, heater 13 comprised of tube 14, heating tape 16 and leads 17, the lower extremity Iof heating chamber 1S, insulating shield 21 land foreign body 28, each of the elements being as hereinbefore described. Free convection current-s, indicated by arrows 29, are generated within the molten zone -bounded by freezing interface 31 and melting interface 32, suc-h currents being directed downwardly in the cooled region adjacent body 28 and upwardly in the hotter region more adjacent to the wall of tube 12. The presence of these currents and their intensity and direction of movement may be made visible by mixing into the crude material 15 a finely divided substance insoluble in at least one of the crude components; for this demonstration, aluminum lactate powder was mixed into naphthalene/zbeta-napthol prior to zonemelting this material. Currents 29 in the present embodiment impinge on the area of the freezing interface 31 and subsequently move through the molten material in the direction of melting interface 32. As previously explained, solute rejected at the growing crystal at the freezing interface is thus transported away from this area which is the desired result.

The desired convection currents may be established by means other than t-he specific structure described above. The hollow tubular foreign body 28 may be replaced, for example, by `a solid rod of conductive material, by an electrically heated element or a refrigerated element. Similarly a plurality of thermal conductors may be utilized and the conductor, or conductors, need not necessarily have the .axial positioning as described herein.

The present invention may be more readily understood by reference to the following example which describes the essentials of a zone-melting operation utilizing a foreign body within the molten zone and is performed in equipment such as that described above. Y

Example To demonstrate the effectiveness of the invention a crude material of predetermined composition was prepared and subjected to a single zone pass operation. A 10% by weight mixture of beta-naphthol in naphthalene was melted and poured into a glass tube twelve inches long and having an internal diameter of 0.69 inch. This corresponds to tube 12 shown in FIGURE l. A stainless tube of the same length and having a 1A; inch outside diameter and a 1%;4 inch Wall thickness served as foreign body 2S and was disposed axially in the still molten beta-naphthol/naphthalene mixture which was then allowed to freeze. A zone chamber and accessory equipment were assembled as shown in FIGURE 1 to supply the essential heating, cooling and movement; the dimensions and materials of construction employed are not critical except to insure steady and measurable heat transfer and movement of tube 12. Filled tube 12 was placed inside the zone chamber and connected remotely :to means exerting a vertical pull. During operation this tube was pulled upward at the rate of 0.18 inch per hour. Tap water at about 20 C. served as a coolant for one chamber while a mixture of approximately 50% by weight ethylene glycol in water at 35 C. was circulated through the other cooling chamber. The 1/2 inch wide heating tape heater produced a molten zone 0.87 inch in length. Tube 1-2 was pulled once through the zone chamber so that substantially all the contents thereof were zone-relined under the above described conditions. At the end of this operation tube 12 was removed from the zone chamber assembly and broken away from the solid container therein. In turn, the stainless steel tubular foreign body was removed by breaking the solid away from it. A spectrophotometric analysis of the solid was performed and it was determined that this single-pass zone refining run had removed 0.67 gram of naphthalene from the recrystallized material and that the diffusion layer was 0.02 inch thick. Calculations based on an experiment carried out under the similar conditions but without the presence of a foreign body showed a naphthalene removal of only 0.38 gram and a diffusion layer thickness of approximately 0.06 inch.

' Comparison of these results shows that the inclusion of a stainless steel tube has reduced the diffusion layer thickness by a factor of three and allowed a 75% increase in separation. Similar benefits, in terms of increased production rate (zone travel rate), are realized in zone-leveling runs where the separation is held constant.

What is claimed is:

1. In the method of zone-melting wherein a molten zone is moved through a solid volume of a fusible substance which is disposed within a long container, the improvement comprising effecting stirring by free convection in said molten zone near the freezing interface thereof, said free convection being provided by disposing a heat conducting foreign body along substantially the entire length of said solid volume of said subtsance, said body being extended within said container along the direction of movement of said molten zone and through the region .to be traversed by said freezing interface, said foreign body being formed of a material which is chemically stable in said molten zone and being maintained at a temperature diering from that of said molten zone.

2. In the method of zone-melting wherein a molten `{zone Vis moved through a solid volume oi' a fusible Vsub`- 'stance disposed withinY a container, the improvement comprising Lreducing the ditusion layer thickness which "resides at the freezing interface of said molten zone, said 1'reduction being accomplished'by providing a long heat conducting foreign body within said solid'volume of said substance whichbody continuously extends through the region thereofto be Vtraversed by said molten zone and which is formed of a material non-reactive with that of said molten zone, said foreign body being aligned along jthe direction of movement of said molten zone within `said container and having a thermal conductivity diiering fromrthat of the moltenzone.

3. In apparatus for performing zone-melting processes,

thecombination comprising a container for holding a solid volume of a substance to be processed, a heating element for producing a molten zone Within said solid volume of` a substance, means for producing a relative Y molten zone and the freezing interface thereof, said body being within said container and within said volume of lsubstance and being aligned along the direction of movement of'said molten zone and extended through substan ltially the entire length of `said volume'of substance, said heat conductiveY body being formed of a material which is chemically stable within said molten Vzone, and'being Y maintained at a temperature differing from-that ofthe Y molten zone whereby convection currents are established'y v comprised of a material having a thermal-conductivity greater than that of rsaid substance within said molten zone.

i References Cited hy the Examiner A UNITED STATES PATENTS Y 2,739,045 3/56 Pfann L 2li-223.5 2,890,940 6/59 Pfann '237301 OTHER REFERENCES ANORlt/IAN YUDKOFF, Primary Examiner.

MAURICE BRINDISI, Examiner. t 

1. IN THE METHOD OF ZONE-MELTING WHEREIN A MOLTEN ZONE IS MOVED THROUGH A SOLID VOLUME OF A FUSIBLE SUBSTANCE WHICH IS DISPOSED WITHIN A LONG CONTANER, THE IMPROVEMENT COMPRISING EFFECTING STIRRING BY FREE CONVECTION IN SAID MOLTEN ZONE NEAR THE FREEZING INTERFACE THEREOF, SAID FREE CONVECTION BEING PROVIDED BY DISPOSING A HEAT CONDUCTING FOREIGN BODY ALONG SUBSTANTIALLY THE ENTIRE LENGTH OF SAID SOLID VOLUME OF SAID SUBSTANCE, SAID BODY BEING EXTENDED WITHIN SAID CONTANER ALONG THE DIRECTION OF MOVEMENT OF SAID MOLTEN ZONE AND THROUGH THE REGION TO BE TRAVERSED BY SAID FREEZING INTERFACE, 